This invention relates to a process for maufacturing soft magnetic components using a ferrous powder and a lubricant, and to compositions produced by the process.
Steel laminations have been used for decades in low frequency magnetic components. The design of stacked magnetic components must take into account the fact that the magnetic flux is confined in planes parallel to the sheet surfaces. Additionally, there are difficulties with miniaturisation and waste material with steel laminations, which can be important for some type of electric motors.
The idea of using iron powder in magnetic components was first introduced by Fritts and Heaviside in the 1880""s. Since the beginning of the century, iron powder has been used for the production of magnetic components (iron powder cores were introduced in the U.S. to replace wire cores around 1915). Powder metallurgy offers the possibility of controlling the spatial distribution of the magnetic flux and allows practically full utilization of materials even for the manufacture of complicated shapes. Recent advances in powder metallurgy offer new opportunities in the design of electromagnetic components. Several authors have shown the advantages to use iron/resin composites especially for applications in the medium and high frequency ranges.
When a magnetic material is exposed to an alternating magnetic field, it dissipates energy. The power dissipated under an alternating field is defined as core losses. The core losses are mainly composed of hysteresis and eddy current losses. Hysteresis losses are due to the energy dissipated by the domain wall movement. The hysteresis losses are proportional to the frequency and are mainly influenced by the chemical composition and the structure of the material.
Eddy currents are induced when a magnetic material is exposed to an alternating magnetic field. These currents lead to an energy loss through Joule (resistance) heating. Eddy current losses are expected to vary with the square of the frequency, and inversely with the resistivity. The relative importance of the eddy current losses thus depends on the electrical resistivity of the material.
Sintered iron powder components are currently used to makc parts for DC magnetic applications. However, sintered parts have low resistivity and are generally not used in AC applications. For applications in alternating magnetic field (AC), a minimal threshold resistivity is required and powder mixes containing insulating resins are generally used. The resin is used to insulate and bind the magnetic particles together. It is well known that iron-resin composites have very low eddy current losses and perform well at moderate and high frequency, while eddy current losses are important at those frequencies in stack assemblies. However, at low frequencies, e.g., 60 Hz, the eddy current portion of the losses is not as important in stack assemblies and the performance of the iron-resin composites is limited by their hysteresis losses. In fact, the hysteresis portion of the losses is higher in iron-resin composite than in stack assemblies. During the fabrication of soft magnetic components with iron powders, stresses are induced in the material. These stresses significantly increase the hysteresis portion of the losses. These stresses can be relaxed by heating the component at high temperature. However, the resin generally used in iron-resin composites cannot withstand the temperature used to relax the stresses. After the thermal treatment, the parts generally do not have sufficient mechanical strength and electrical resistivity for many applications.
Powder formulations for the fabrication of annealed soft magnetic components for AC soft magnetic applications have been described in patent literature.
U.S. Pat. No. 5,595,609 issued Jan. 21, 1997 to Gay discloses polymer-bonded soft magnetic body that can be annealed at temperature around 500xc2x0 C. The magnetic powder used is encapsulated with a thermoplastic coating selected from the group of polybenzimidazole and polyimides having heat deflection temperatures of at least about 400xc2x0 C.
U.S. Pat. No. 5,754,936 issued May 19, 1998 to Jansson, and WO 95/29490 disclose phosphate coated powders that can be used for the fabrication of annealed components. After compaction, the components are treated at temperature ranging from 350 to 500xc2x0 C. to stress relief the magnetic powders.
U.S. Pat. No. 5,352,522 issued Oct. 4, 1994 to Kugimiya et al. discloses oxide coated powders that can be processed at high temperature (800xc2x0 C.) for the fabrication of soft magnetic components.
European patent application EPO 088 992 A2 discloses oxide coated powders for the fabrication of magnetic components processed at high temperatures (900xc2x0 C.).
U.S. Pat. No. 4,601,765 issued Jul. 22, 1986 to Soileau et al. discloses silicate coatings for the fabrication of annealed components.
F. Hanejko et al. xe2x80x9cApplication of High Performance Material Processing Electromagnetic Productsxe2x80x9d in the Proceedings of the 1998 International Conference on Powder Metallurgy and Particulate Materials, May 31-Jun. 4th, Las Vegas, Nev., 1998,p. 8-13. presented results on annealed soft magnetic components fabricated with coated powders.
The above-discussed prior art discloses coated powders for the fabrication of annealed soft magnetic components for AC soft magnetic applications. Coating the powder represents an additional step during the preparation of the material. It involves additional cost and the preparation of the powder may require additional equipment. None of the prior art discloses compositions produced with uncoated powders. In addition, in most of the prior art processes, the composition does not contain an admixed lubricant and cannot be processed using simple compaction at room temperature without using die wall lubrication.
Other references of interest are: R. W. Ward and D. E. Gay, xe2x80x9cComposite Iron Materialxe2x80x9d, U.S. Pat. No. 5,211,896 (1993); H. Rutz and F. G. Hanejko, xe2x80x9cDoubly-Coated Iron Particlesxe2x80x9d, U.S. Pat. No. 5,063,011 (1991); G. Katz, xe2x80x9cPowdered Iron Mapetic Core Materialsxe2x80x9d, U.S. Pat. No. 2,783,208 (1957); and P. N. Roseby, xe2x80x9cMagnet Corexe2x80x9d, U.S. Pat. No. 1,789,477 (1931). This prior art does not refer to iron-lubricant mixes that are treated at moderate temperature to partly eliminate the lubricant without sintering to maintain an adequate electrical resistivity.
Mixes (compositions) composed of iron powder and lubricant have been used for a long time for powder metallurgy applications. The lubricant is used to ease the compaction of the powder, ease the ejection of the part from the die and to minimize die wear. After compaction, the part does not have sufficient mechanical propertics and must be sintered to create metallurgical bonds between the particles. Sintering is generally done at temperature ranging from 1000xc2x0 C. up to 1200xc2x0 C. Specimens compacted from iron-lubricant mixtures cannot be used in the green (non-heated) nor the sintered state for the fabrication of components for AC soft magnetic applications, having low core losses. The green parts do not have sufficient mechanical strength while the sintered components do not have sufficient electrical resistivity to maintain low eddy current losses.
It is an object of the present invention to provide powder compositions and a process for the fabrication of soft magnetic components intended for low frequency soft magnetic applications.
It is a further object of this invention to increase the mechanical strength of the components without sintering.
It has been found that non-coated iron powder admixed with a lubricant can be used for the fabrication of soft magnetic components having low core losses at low frequency. According to the invention, the non-coated powder is mixed with a solid lubricant. After compaction, the specimens are heated at a moderate temperature, below the level corresponding to full sintering.
The thermal treatment removes, to a large degree, the lubricant. Bonds between the powder particles, which may have a positive effect on the mechanical strength of the material, may be created during the thermal treatment. If the material does not have sufficient mechanical strength after the thermal treatment, the material may be impregnated with a resin to further increase the mechanical strength.
At higher temperatures, typically above about 400xc2x0 C., the thermal treatment relieves the internal stresses induced during the compaction. However, the advantages are still present when the heat treatment is effected at lower temperatures, for example as low as 300xc2x0 C. Between 300xc2x0 and 400xc2x0 C., little or no stress relief takes place. The process even at lower temperatures produces a powder that is easy to prepare (there is no need to coat the particles), the powder can be compacted without using die wall lubrication, the material has low loss in AC magnetic applications and acceptable mechanical properties. Also, there is the advantage that the electrical resistivity is higher. This can be an important advantage in practice.
The soft magnetic powder is not coated before mixing with the lubricant. The non-coated powder is admixed with a lubricant. The lubricant prevents the formation of interparticle contacts during compaction and may leave residues after delubrication, which increase the electrical resistivity of the material. The powder is compacted using conventional powder metallurgy techniques. Since the powder contains an admixed lubricant, the powder can be shaped without using die wall lubrication. The properties of the material may be adjusted by modifying the lubricant type and content and the thermal treatment conditions. The processing conditions described in the present application allow obtaining material with low core losses.
The powder composition comprises a ferromagnetic powders, such as pure iron or iron alloy powder. The typical average particle size of the starting powder can range from 5 xcexcm to 1 mm, but preferably below 250 xcexcm or 60 US mesh. In the tests conducted to validate the invention, the powder used was ATOMET(trademark) 1001HP water-atomized iron powder designed for soft magnetic P/M applications available from Quebec Metal Powders Limited, Tracy, Quebec, Canada.
The ferromagnetic powder is admixed with a lubricant. The admixed lubricant reduces the friction between the compacts and die walls and minimize die wear during compaction of the component. The lubricant prevents the formation of interparticle electrical contacts during compaction and increases significantly the electrical resistivity of the green (as-pressed) material. The lubricant may be any lubricant known for powder metallurgy applications. The lubricant may be, for example, selected from synthetic waxes, amide-based waxes, metallic stearates, polymeric lubricants, fatty acids, boric acid or borate esters. The lubricant may be dry-mixed with the powder, or it may be melted or dissolved for admixing. The lubricant may also be bonded to the iron based powder with a binder. The choice of the lubricant will mainly depend on the required properties of the material. Some lubricants provide parts with higher electrical resistivity after the thermal treatment, while other lubricants provide parts witb higher permeability or higher mechanical strength. The amount of lubricant also depends on the required properties of the final material. Increasing the amount of lubricant improves the electrical resistivity after thermal treatment, but lowers the permeability. The amount of lubricant should be typically between 0.25 wt % and 4 wt %, but preferably between 0.5 wt % and 2.0 wt % of the powder-lubricant mixture.
The powder is compacted or molded into the desired component or shape. Generally, the method used to consolidate metal powders into integral components consists of filling the die with the powder and pressing the powder at the appropriate pressure and temperature. Pressing the parts at higher pressure and temperature increases the density and consequently the permeability. However, increasing the compacting pressure and temperature reduces by the same way the electrical resistivity of the compacts and consequently increases the eddy currents in the parts as frequency increases.
After compaction, the specimens undergo a thermal treatment at a moderate temperature. The thermal treatment is earned out to burn out the lubricant and in soemcases to stress relief the parts. Thermal oxidation bonding between the ferromagnetic particles may occur during the thermal treatment. Lubricant decomposition products may also form interparticular bonds during the thermal treatment and increase the mechanical strength.
In order to maintain sufficient electrical resistivity and to stress relief the components, the thermal treatnent should be effected at temperatures ranging from 300xc2x0 C. to 700xc2x0 C. The temperature should be selected such as to avoid sintering of the powder at least to a substantial degree. The thermal treatment duration may vary from 1 min up to 6 hours but preferably between 1 and 30 min. The thermal treatment conditions are generally chosen to optimize the magnetic properties of the component. Increasing thermal treatment temperature and duration generally lowers the electrical resistivity and the hysteresis portion of the losses. By optimizing the thermal treatment conditions, it is possible to reduce the total core losses.
If the mechanical strength of the treated components is not sufficient, the components may be impregnated to increase their mechanical strength. The impregnation should be carried out after the thermal treatment. The impregnant can be selected from the group consisting of thermosetting and thermoplastic resins, low-melting point inorganic insulators or the precursors of the latter. The only limitation on the choice of the impregnant, which must of course be electroinsulative, is its ability to flow around each ferromagnetic particles and pores and increase the mechanical strength of the parts. The impregnant can be melted or dissolved in a compatible solvent prior to the impregnation. The impregnation can be done at room temperature or with heating and under atmospheric pressure. The impregnation can also be done under pressure optionally with heating to make the impregnation easier. Depending on the type of binder used, a heat treatment or curing can be done after the impregnation.
A particularly interesting feature of the present invention is that the powder can be shaped at room temperature using conventional powder metallurgy techniques. The powder can be shaped without die wall lubrication, since the powder mix contains an admixed lubricant. The formulation is easy to prepare since the powder does not have to be coated with an inorganic insulative coating prior to compaction. Parts fabricated according to the methods described above have sufficient electrical resistivity and low core losses at 60 Hz. The parts also present mechanical strength sufficient for many soft magnetic applications.